New Mexico Geological Society Annual Spring Meeting — Abstracts

The Precambrian rocks in the Hopewell Lake area are in stratigraphic order: the Moppin metavolcanic series, the Burned Mountain
metarhyolite and a succession of metasediments. The Moppin series is intruded by a granodiorite-tonalite pluton and consists of mafic felsic volcanic and volcaniclastic rocks. The Burned Mountain metarhyolite contains relict eutaxitic textures. On a Zr/TiO₂ versus Nb/Y plot (after Winchester and Floyd, 1977), rocks of the Moppin series plot in the fields of dacite, andesite and basalt; the Burned Mountain metarhyolite plots in the field of rhyolite. All of the Precambrian rocks have been metamorphosed and display either a dominant or ductile deformation fabric.

Gold mineralization is hosted principally by the Moppin series rocks. Gold grades are 1 to 1160 ppm in the altered rocks, and 0.1 to 4 ppb in the unaltered rocks. Gold is associated with chalcopyrite and pyrite, occurring typically as minute disseminations within these sulphide phases. Other ore minerals are sphalerite, galena, arsenopyrite and hematite. Intense ferroan carbonate alteration commonly characterize the mineralization.

Homogenization temperatures obtained for CO₂-rich and aqueous-rich inclusions span over a range of 250 to 330°C. Calculated pressure of entrapment for these inclusions is 1.5Kb. Rb-Sr isotopic analyses of seven samples from the granodiorite-tonalite intrusive define an isochron with a date of 1467 Ma ± 43 Ma and an initial 87Sr/86Sr ratio of 0.70256 ± 0.00029. On an evolutionary diagram, the isotopic composition of all samples of the intrusive rock were found to have been reset between 1400 and 1560 Ma. All the intrusive samples also have negative £_Sr (1460), indicating they were depleted relative to bulk earth.

The Moppin series represents a continuous calcalkaline volcanic suite comparable to modern volcanic suites forming along converging plate margins. The Burned Mountain metarhyolite probably represents a subaerial ash-flow tuff. The granodioritetonalite intrusive was derived directly or indirectly from a depleted mantle source. The date of 1467 Ma represents either the age of the intrusive or that a thorough alteration event which affected it. The preferred model is that the mineralizing fluids, and possibly the gold, were derived from a depleted mantle source.